A fixing apparatus heretofore known as an induction heating (IH) type of fixing apparatus has a thin-walled heat-producing rotating element including a conductive layer that produces heat by means of induction heating and is installed in a rotatable fashion, an induction heating source comprising a magnetic flux generation section that is located opposite the outer surface of the heat-producing rotating element and induction-heats the heat-producing rotating element, a rotatable internal pressure member that is in contact with the inner surface of the heat-producing rotating element, and a rotatable external pressure member that is in contact with the outer surface of the heat-producing rotating element (see, for example, Patent Document 1).
FIG. 1 is a schematic cross-sectional drawing of a fixing apparatus disclosed in the Patent Document 1. As shown in FIG. 1, this fixing apparatus has a coil assembly 10 that generates a high-frequency magnetic field as the induction heating source, a metal sleeve 11 as the heat-producing rotating element that produces heat through induction heating by means of coil assembly 10 and is rotatably installed, a rotatable internal pressure member 12 that is in contact with the inner surface of metal sleeve 11, and a rotatable external pressure member 13 that is opposite internal pressure member 12 and is in contact with the outer surface of metal sleeve 11.
In FIG. 1, metal sleeve 11 is gripped between external pressure member 13 and internal pressure member 12, and rotates driven by the rotation of external pressure member 13.
Recording material 14 to which an unfixed toner image has been transferred is transported from the direction indicated by the arrow toward a nip area 23 formed between external pressure member 13 and metal sleeve 11. In nip area 23, heat of metal sleeve 11 heated by coil assembly 10 and pressure from both internal pressure members 12 and 13 are applied to recording material 14. By this means, an unfixed toner image is fixed onto recording material 14.
Metal sleeve 11 of this fixing apparatus is a flexible, thin, hollow metal conductor with a thickness of 20 μm to 60 μm, and includes a conductive layer formed of an electrically conductive magnetic material such as nickel, iron, or SUS430.
Coil assembly 10 of this fixing apparatus is supported by a holder (not shown) and fixed to a fixing unit frame at a predetermined distance from the outer surface of metal sleeve 11, and performs Joule heating of metal sleeve 11 opposite by inducing an induction current (eddy current) in metal sleeve 11.
In this fixing apparatus, since metal sleeve 11 is heated by means of coil assembly 10 located on the outside of metal sleeve 11, an excessive rise in the ambient temperature due to heat production by coil assembly 10 itself and thermal radiation to metal sleeve 11 can be reduced. Also, in this fixing apparatus, since an unfixed toner image is heat-fixed onto recording material 14 by heating metal sleeve 11 directly, there is little loss of heat from metal sleeve 11 during warming-up compared with a fixing apparatus in which metal sleeve 11 is heated indirectly by a supporting roller, for example. Moreover, since metal sleeve 11 is thin, the heat capacity of metal sleeve 11 itself is small, and startup responsiveness until metal sleeve 11 is heated to a predetermined fixing temperature is improved.
Also, a fixing apparatus is known that has a thin heating belt that includes a conductive layer, a magnetic field generation section that performs induction heating of that conductive layer from outside the thin heating belt, and a ferromagnet through the gap with respect to the heating belt on the opposite side of the heating belt from the magnetic field generation section, and fixes an unfixed toner image on a recording medium in a nip area between the heating belt and a pressure member located opposite (see, for example, Patent Document 2).
In this fixing apparatus, the heat capacity of the heating member is extremely small, and the warm-up time is shortened. Also, ample heat and pressure can be applied in the nip area between the heating belt and the pressure member, so that good fixability can be obtained.    Patent Document 1: Japanese Patent Application Laid-Open No. HEI10-74007    Patent Document 2: Japanese Patent Application Laid-Open No. 2004-145368